Polarization effects on microwave imaging of dielectric cylinder

Theoretical and experimental studies are presented of frequency-swept microwave imaging of an infinitely long lossless homogeneous dielectric cylinder not satisfying the Born approximation and illuminated by a right-hand circularly polarized plane wave. The reconstructed polarization-dependent microwave image is seen as embodying contributions from specular, axial, glory, and stationary ray components of the scattered field of the selected receiving polarization state. An automated microwave imaging system using frequency and polarization diversity techniques is used to verify the theoretical and numerical results     

Generalized Rational Function Approximation in Finite Intervals Using Zolotarev Functions

The results of Chebyshev in the theory of approximation find wide application in the design of circuits and microwave components. Part of the comparatively unknown work of Zolotarev and Achieser which has found useful applications in the design of certain microwave components, especially of mixed lumped/distributed components, is described. Several new results are presented, including formulas which enable the approximation functions to be computed in a relatively simple manner.     

On the numerical analysis of the subreflector of an offset Cassegrain microwave antenna

The physical optics analysis of the scattering from the subreflector of an offset Cassegrain microwave antenna is formulated in a novel way using the geometrical properties of the conic sections. The resulting integral is computed by means of an algorithm involving quadratic approximation of the phase and polynomial approximation of the remainder of the integrand. The formulation allows for displacement of the phase center of the feed pattern representation. Examples are presented which provide a comparison with previously reported results obtained using the geometrical theory of diffraction rather than physical optics.     

Two-dimensional microwave imaging approach based on a geneticalgorithm

A multi-illumination multiview approach to microwave imaging is proposed, which is based on a genetic algorithm (GA). The inverse problem is recast in an optimization problem, solved in the framework of the Born approximation. The formulation of the approach is described and images of reconstructed circular (multilayer) cylinders are reported     

An improved closed-form approximation to the inverse black body radiation problem at microwave frequencies

A function is derived which approximates Planck's black body radiation law more closely at microwave frequencies than either the Rayleigh-Jeans or the recent Hamid-Ragheb approximations. The function leads to an improved closed-form approximation to the inverse black body radiation problem in the microwave region.     

Microwave-Induced Thermoacoustic Effect in Dielectrics and its Coupling to External Medium--A Thermodynamical Formulation

A thorough formulation of electromagnetic wave interaction with biological systems is presented. The thermodynamic process of the microwave-induced thermoacoustic generation is clearly defined. Couplings of the acoustic and thermal energies to the surrounding medium are included through consideration of discontinuities of thermodynamical variables and microwave exposure. Contrary to prior analyses, it is shown that acoustic waves may be generated by pulsed microwaves, even in the absence of inhomogeneity of microwave absorption, owing to discontinuities of thermodynamical variables and microwave exposure conditions across the interface. General equations for the thermoacoustic waves are derived, and the validity of the first-order linear approximation is estimated in terms of its percentage error. For a system with water as the absorbing dielectric interfacing with air of 1 atmosphere pressure, the first-order approximation becomes invalid for a peak specific absorption rate greater than 13 kW/gm.     

An inexact-Newton method for short-range microwave imaging within the second-order Born approximation

A new approach to noninvasive inspection of dielectric targets at microwave frequencies is proposed. Cylindrical dielectric objects are reconstructed under the second-order Born approximation. A multi-illumination configuration is considered. The continuous model is discretized by the moment method and an efficient inexact-Newton method is applied. The dielectric profile is iteratively reconstructed starting from the measured scattered data, which are related to the unknown target through the inverse scattering equations written in a variational setting. Several numerical results are reported, which are aimed at assessing the capabilities of the approach in dealing with the nonlinear ill-posed inverse problem associated to the short-range microwave imaging. Single, multilayer, and separate cylinders are reconstructed in noiseless and noisy environments.     

New Method for Computing the Resonant Frequencies of Dielectric Resonators (Short Papers)

A new method is develop for accurately predicting resonant frequencies of dielectric resonators used in microwave circuits. By introducing an appropriate approximation in the field distribution outside the resonator an analytical formulation becomes possible. Two coupled eigenvalue equations thus derived are subsequently solved by a numerical method. The accuracy of the results computed by the present method is demonstrated by comparison with previously published data.     

Design procedure for inhomogeneous coupled line sections

Design formulas are presented, along with a procedure for the design of inhomogeneous coupled line sections as an approximation to a series open-circuited stub for application in the realization of microwave pseudo-high-pass filters. The accuracy of the design equations is evaluated through the design and test of a seventh-order filter and it is found that the formulation predicts the performance of the section well beyond the quarter-wave frequency     

Novel applications of an approximate profile inversion forone-dimensional medium

In one-dimensional inverse scattering, the approximate formulas to reconstruct the permittivity profiles are very interesting because of their closed forms. Nevertheless, as the result of Born approximation, they are invalid under the strong-scattering conditions which restrict their applications. In this paper, a novel approximation for inverting the permittivity profile of a one-dimensional inhomogeneous medium is derived by using an equivalent microwave networking technique. Compared with the published formulas of Ladoucer and Jordan (1985), the novel approximation is not only more general in the form, but suitable for both weak-scattering and strong-scattering conditions. Reconstruction examples show that when the relative permittivity reaches 80, the novel formula is effective, which show the validity of this scheme     

Frequency-swept microwave imaging of dielectric objects

Analytical and experimental studies are presented of frequency-swept microwave imaging of a nondispersive dielectric object satisfying the Born approximation. The retrieved images show from experimental data measured in the frequency range 6-17 GHz are free of the speckle noise that plagues conventional coherent imaging system. The results demonstrate that the microwave imaging system has potential as a cost-effective tool in nondestructive evaluation of dielectric objects     

Conformal mapping of the field and charge distributions inmultilayered substrate CPWs

Conformal mapping (CM) and partial capacitance techniques are used for analytical evaluation of charge/current and electric- and magnetic-field distributions in a multilayered substrate coplanar waveguide (CPW) in the quasi-TEM approximation. The results, compared with finite-element method simulations, show that the magnetic wall assumed at the dielectric-dielectric interfaces in CM is a good approximation for many practical cases. The method is applied to a CPW with a thin ferroelectric film used in tunable microwave devices     

Microwave remote sensing of a two-layer random medium

A two-variable expansion technique is used to solve for the mean Green's function from the Dyson equation under the nonlinear approximation. The Bethe-Salpeter equation then gives rise to a set of modified radiative transfer (MRT) equations which accommodate coherent effects essential to bounded media. It is found that the nonlinear approximation, instead of the more popular bilocal approximation, should be used for the case of bounded media. The two approximations yield identical results for unbounded media. The MRT equations are then solved for a two-layer random medium. The MRT equations give rise to simple and useful solutions which are applicable to both active and passive microwave remote sensing.     

Calculation of Microwave Attenuation Effect Due to Charged Sand Particles

Based on the forward scattering amplitude function for charged sand particles under the Rayleigh approximation and the effective permittivity method, a calculation model for microwave attenuation due to charged sand particles is given in terms of equal sized distribution and lognormal size distribution, and the attenuation is calculated and analyzed. The results show that the attenuation with charged sand is greater than the case of no charge, and the more concentrative the surface charges on sand particles are, the greater are the influences on microwave attenuation. When the frequency is not too high, natural sand and vehicular sand have little effect on microwave attenuation, whereas the attenuation of explosive sand need be considered.     

Theoretical Study of a Two-Section Ultrawideband Antenna Array

The paper presents a theoretical consideration of a two-dimensional array consisting of two series-connected parts: an infinite system of one-dimensional arrays of strip horns and an array of planar horns. The eigenwave problem for an infinite one-dimensional array of strip transmission lines k]which reduces to a singular integral equation with respect to the current on the strip k]is solved. An analytical solution for the current and an expression for the characteristic impedance of the principal T-wave of the array k]which is valid in the scanning mode k]are found. A model of an array with smoothly varying parameters that describes the operation of a one-dimensional array of strip horns in the approximation of a microwave transmission line is developed. In the same approximation k]a model of an array of planar horns and the entire two-dimensional two-section array is developed and its numerical study is performed.     

Methods of calculation of nonstationary nonharmonic fields in guiding electrodynamic structures

Methods of calculation of nonstationary nonharmonic electromagnetic fields excited by electron streams in guiding electrodynamic structures (ESs) of microwave devices are considered. The methods can be applied in first- and second-level spectral models. The proposed discrete approximation is based on decompositions of the field in normal or partial modes of ESs. The continuous approximation proposed is based on decomposition of the field in regular modes of ESs.     

WKB近似下的Fourier衍射成象方法

对于介质目标微波衍射成象，本文引入了ＷＫＢ近似来模拟目标内部总场。基于这种近似，我们导出了Ｆｏｕｒｉｅｒ衍射公式，并采用了广义滤波逆传播方法由目标空间谱实现目标特性的重建。计算机模拟结果表明采用ＷＫＢ近似重建目标特性较Ｂｏｒｎ近似有明显改善。     

Hot electron microwave conductivity of wide bandgap semiconductors

Hot electron microwave conductivity of the wide bandgap semiconductors GaN, SiC and Diamond has been calculated using displaced Maxwellian approximation for the electron distribution function. The effects of both the energy and momentum relaxation times due to scattering by acoustical, optical intervalley phonons and by ionized impurities are included in the derivations. Numerical results for the microwave conductivity and the change in dielectric constant as a function of frequency and bias electric field are presented. It is found that significant change in the conductivity and dielectric constant contribution for a fixed bias field occurs at very high frequencies on the order of 10¹² Hz, which is well beyond the range of current microwave device interest.     

Image Parameter Design of Noncommensurate Distributed Structures: An Application to Microstrip Low-Pass Filters

The direct application of the image parameter method (IPM) to distributed structures is suggested in order to overcome some limitations of classical design methods for microwave filters. Several advantages are pointed out: 1) Wider degrees of freedom are obtained using noncommensurate structures. 2) The IPM can be applied directly to a microwave structure without any use of lumped prototypes. 3) Possible technological constraints can be easily incorporated in the design procedure. 4) Filters designed according to the IPM can be cascaded together in order to improve their characteristics. An application to the design of a class of microstrip low-pass filters, which have been previously designed on a low-frequency approximation basis, is illustrated in detail. The IPM is shown to provide an effective control of both the passband and stopband, leading to filters with improved characteristics, as demonstrated by the experimental results.